An antenna array usually comprises a plurality of antenna elements situated side by side with a common distribution network connecting the individual elements to a feed point through which the electromagnetic field is fed at a given microwave frequency, e.g. within the X band. The antenna elements may comprise centrally fed waveguides provided with radiation openings in the form of slits along the side surface opposite the feed opening. the U.S. Pat. Nos. 3,363,253 and 4,429,313 illustrate examples of such an antenna in a resonant implementation, i.e. where a slitted wave guide is short-circuited at its ends, and where the slits are placed exactly half a wavelength (.lambda.g/2) from each other, thus obtaining a standing wave. An antenna array of this kind generally has the advantage that it may be controlled electrically, i.e. the direction of the main lobe of the antenna may be varied by varying the phase of the electromagnetic field fed to the individual antenna elements. A disadvantage with a resonant-type antenna is its very restricted bandwidth properties.
Another type of wave guide antenna element is a non-resonant element provided with an absorbent termination, and where the slits have mutual spacing differing somewhat from half the wavelength (.lambda.g/2), a propagating wave thus being obtained. C.f. R. C. Hansen, "Microwave Scanning Antennas", Part III. In this type of element the lobe is directed at a given angle to the normal. On a change of the frequency of the energy fed to the element via the feed opening the lobe moves in relation to the normal of the element, however, i.e. the lobe direction varies with the frequency, making the antenna array unusable in many applications, unless special measures are taken.